The invention relates to a process for removing impurities from an oxide of titanium-bearing material and, in a preferred embodiment, a process for upgrading ilmenite ore to synthetic rutile by the action of sulfur trioxide.
Titanium ores having commerical importance presently include rutile, which contains about 95% by weight titanium dioxide (TiO.sub.2), and ilmenite ore which is nominally ferrous titanate containing about 45 to 65% by weight TiO.sub.2. These two ores can be used in the production of TiO.sub.2 suitable as pigments and fillers in paper and plastics with lesser amounts being used in metal production.
Production of TiO.sub.2 pigment can be carried out by the sulfate process, in which ilmenite or titania-enriched smelter slag is reacted with concentrated sulfuric acid, or by the chloride process in which rutile or ilmenite is chlorinated to produce titanium tetrachloride which is reoxidized to TiO.sub.2. The second process is preferred in that chloride plants employing rutile feed stock are less expensive and less waste problems are encountered as compared with production plants using the sulfate process. However, increased consumption of rutile in chloride-process pigment manufacture has placed increased demands upon the limited reserves of rutile, thereby increasing costs and rendering the process less economical.
Thus, there has been considerable interest in upgrading ilmenite to produce a synthetic rutile which can be substituted as the feed stock in existing fluidized bed chlorinators employing the chloride process for the production of titanium tetrachloride from rutile.
In U.S. Pat. No. 3,996,332, of which one of the present inventors was an inventor, a process is described whereby ilmenite is smelted to produce a slag which is then oxidized, with a titanium pyrophosphate flux in order to produce a synthetic rutile.
U.S. Pat. No. 3,560,153 describes a process whereby ilmenite is reacted with sulfur dioxide and a reductant in order to convert the ion present into the corresponding sulfide. The thus-treated ilmenite can then be leached, leaving behind the titanium dioxide.
In U.S. Pat. No. 3,860,412, a process is described where by a titaniferous material is upgraded into a feed stock suitable for the preparation of titanium tetrachloride by the direct chlorination method whereby the titaniferous material is communinuted, mixed with sulfuric acid and heated.
U.S. Pat. No. 3,252,787 covers a process for the production of a high quality titanium dioxide concentrate from a titaniferous ore, particularly ilmenite, by roasting the ore in air to convert the iron to the ferric state, reducing the ferric iron to metallic iron with a reducing agent and leaching out the metallic iron.
Further, titanium sulfide can be produced from "black sands" by heating with sulfur dioxide and a hydrocarbon gas as described in U.S. Pat. No. 3,313,601.
Generally, research on the upgrading of ilmenite involves the removal of iron by hydrometallurgical, pyrochemical or pyrometallurigical methods. These processes include the prereduction of ilmenite followed by 2-stage sulfuric acid leaching of the iron therefrom, the pre-reduction of ilmenite followed by reoxidation of metallic iron in a water slurry and separation of the hydrated iron oxides and, finally, a process based upon the selective chlorination of iron in ilmenite to separate iron chloride leaving rutile as the residue. However, such processes have not been particularly successful in utilizing the more abundant low grade ilmenite ores to produce a substitute chlorination feed stock which approaches natural rutile in TiO.sub.2 content. Most of these processes are applicable to the use of high grade ilmenite ores from deposits of limited reserves.
Thus, various processes for the upgrading of ilmenite to synthetic rutile employ the initial conversion of the ilmenite into a titania slag, see especially U.S. Pat. Nos. 3,860,412 and 3,996,332. However, commercially produced titania slag contains associated alkaline earth impurities. These alkaline earth impurities form high boiling point chloride mixtures which can accumulate and cause defluidization of the bed in the subsequent chlorination reaction of the synthetic rutile to give titanium tetrachloride. In addition, these alkaline earth chloride mixtures form a liquid coating on the surface of the particles to be chlorinated, thus causing impeaded chlorination of the TiO.sub.2 and an inefficient use of the chlorine.
It is therefore an object of the present invention to remove impurities, particularly alkaline earth impurities, from an oxide of titanium-bearing material.
It is a further object of the present invention to upgrade ilmenite in order to produce a synthetic rutile.
An additional object of the present invention is to upgrade ilmenite ore to produce a feed stock suitable for use in a chlorination reactor for the conversion of rutile to titanium tetrachloride.
An additional object of the present invention is to directly convert a titania slag to a feed stock suitable for use in a chlorination reactor for the conversion of rutile to titanium tetrachloride.